Low torque engine starting with dual spool power extraction with superposition gearbox

ABSTRACT

A disclosed turbofan engine includes a first spool including a first turbine; a second spool including a second turbine disposed axially forward of the first turbine, a first tower shaft engaged to the first spool, a second tower shaft engaged to the second spool, and a superposition gearbox including a sun gear, a plurality of intermediate gears engaged to the sun gear and supported in a carrier and a ring gear circumscribing the intermediate gears. The second tower shaft is engaged to drive the sun gear. A starter is selectively coupled to the sun gear through a starter clutch. A first clutch selectively couples the first tower shaft to the ring gear and a second clutch selectively couples the ring gear to a static structure of the engine. An accessory gearbox is driven by an output of the superposition gearbox.

BACKGROUND

A gas turbine engine typically includes a fan section, a compressorsection, a combustor section and a turbine section. Air entering thecompressor section is compressed and delivered into the combustionsection where it is mixed with fuel and ignited to generate ahigh-energy exhaust gas flow. The high-energy exhaust gas flow expandsthrough the turbine section to drive the compressor and the fan section.The compressor section typically includes low and high pressurecompressors, and the turbine section includes low and high pressureturbines.

A high pressure turbine drives the high pressure compressor through anouter shaft to form a high spool, and the low pressure turbine drivesthe low pressure compressor through an inner shaft to form a low spool.The fan section may also be driven by the low pressure turbine throughthe inner shaft.

The engine is typically started by driving the high spool through atower shaft with a starter through an accessory gearbox. Once the highspool is up to speed, the low spool follows and the engine is brought toan idle condition. When the engine is operating, the accessory gearboxis driven through the same tower shaft to drive accessory componentssuch as hydraulic pumps and electric generators.

Turbine engine manufacturers continue to seek further improvements toengine performance including improvements to thermal, transfer andpropulsive efficiencies.

SUMMARY

A turbofan engine according to an exemplary embodiment of thisdisclosure includes, among other possible things, a first spoolincluding a first turbine; a second spool including a second turbinedisposed axially forward of the first turbine; a first tower shaftengaged to the first spool; a second tower shaft engaged to the secondspool; a superposition gearbox including a sun gear, a plurality ofintermediate gears engaged to the sun gear and supported in a carrierand a ring gear circumscribing the intermediate gears, wherein thesecond tower shaft is engaged to drive the sun gear; a starterselectively coupled to the sun gear through a starter clutch; a firstclutch for selectively coupling the first tower shaft to the ring gear;a second clutch for selectively coupling the ring gear to a staticstructure of the engine; and an accessory gearbox driven by an output ofthe superposition gearbox.

In a further embodiment of the foregoing turbofan engine, the output ofthe superposition gearbox comprises a carrier shaft attached to thecarrier.

In another embodiment of any of the foregoing turbofan engines, thesuperposition gearbox includes the carrier shaft attached to the carrierto drive gear systems within the accessory gearbox, and a sun gear shaftthat is coupled to the sun gear, the second tower shaft, and the starterthrough the starter clutch.

In another embodiment of any of the foregoing turbofan engines, thefirst turbine comprises a low pressure turbine and the second turbinecomprises a high pressure turbine.

In another embodiment of any of the foregoing turbofan engines, thefirst tower shaft and the second tower shaft are concentric about acommon axis.

In another embodiment of any of the foregoing turbofan engines, thefirst tower shaft and the second tower shaft are disposed aboutdifferent axes.

In another embodiment of any of the foregoing turbofan engines, thestarter clutch, first clutch and the second clutch comprise one-waymechanical clutches.

In another embodiment of any of the foregoing turbofan engines, thefirst clutch couples the first tower shaft to the ring gear and thesecond tower shaft drives the sun gear in a first operating conditionsuch that both the first tower shaft and the second tower shaft combineto drive the output.

In another embodiment of any of the foregoing turbofan engines, thesecond clutch couples the ring gear to the static engine structure. Thestarter clutch couples the starter to the sun gear and the first clutchcouples the first tower shaft to the ring gear such that the starterdrives the sun gear and thereby the second tower shaft and the secondspool in a starting operating condition.

In another embodiment of any of the foregoing turbofan engines, thesecond clutch decouples the ring gear from the static structure andcouples the first clutch in response to rotation of the first towershaft driven by the first spool.

An auxiliary gearbox drive system for a turbofan engine according to anexemplary embodiment of this disclosure includes, among other possiblethings, a sun gear, wherein the sun gear is coupled to a second towershaft of the turbofan engine; a plurality of intermediate gears engagedto the sun gear and supported in a carrier; a ring gear circumscribingthe intermediate gears; a starter selectively coupled to the sun gearthrough a starter clutch; a first means for selectively coupling thering gear to a first tower shaft of the turbofan engine; a second meansfor selectively coupling the sun gear to a fixed structure; and anoutput to an auxiliary gearbox.

In a further embodiment of the foregoing auxiliary gearbox drive systemfor a turbofan engine, the output to the auxiliary gearbox comprises acarrier shaft attached to the carrier.

In another embodiment of any of the foregoing auxiliary gearbox drivesystems for a turbofan engine, each of the first means for selectivelycoupling the ring gear to the first tower shaft and the second means forselectively coupling the ring gear to the fixed engine structure areone-way mechanical clutches.

In another embodiment of any of the foregoing auxiliary gearbox drivesystems for a turbofan engine, the first means couples the first towershaft to the ring gear and the second tower shaft drives the sun gear ina first operating condition such that both the first tower shaft and thesecond tower shaft combine to drive the carrier shaft.

In another embodiment of any of the foregoing auxiliary gearbox drivesystems for a turbofan engine, the second means couples the ring gear tothe fixed structure when the starter is driving the sun gear to rotatean engine spool through the second tower shaft in a starting operatingcondition.

A method of operating an auxiliary gearbox for a turbofan engineaccording to an exemplary embodiment of this disclosure includes, amongother possible things, coupling a first tower shaft to engage a firstspool; coupling a second tower shaft to engage a second spool; couplinga sun gear of a superposition gearbox to the second tower shaft, whereinthe superposition gearbox includes the sun gear, a plurality ofintermediate gears engaged to the sun gear and supported in a carrierand a ring gear circumscribing the intermediate gears; selectivelycoupling a starter to the sun gear; and selectively coupling the ringgear to an engine static structure; and driving the second spool withthe starter through the sun gear to rotate the second spool and startthe turbofan engine.

In a further embodiment of the foregoing method of operating anauxiliary gearbox for a turbofan engine, coupling the first tower shaftis coupled to the ring gear such that the first tower shaft drives thering gear and the second tower shaft drives the sun gear once both thefirst spool and the second spool are rotating independent of rotation ofthe starter. Both the first tower shaft and the second tower shaftcombine to drive an output shaft from the superposition gearbox.

In a further embodiment of any of the foregoing methods of operating anauxiliary gearbox for a turbofan engine, the output shaft is attached tothe carrier and drives gear systems disposed within an accessorygearbox.

Although the different examples have the specific components shown inthe illustrations, embodiments of this invention are not limited tothose particular combinations. It is possible to use some of thecomponents or features from one of the examples in combination withfeatures or components from another one of the examples.

These and other features disclosed herein can be best understood fromthe following specification and drawings, the following of which is abrief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example gas turbine engine embodiment.

FIG. 2 is a schematic representation of the example accessory drivesystem.

FIG. 3 is a schematic illustration of the accessory drive system in astarting operating condition.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. The fan section 22 drivesair along a bypass flow path B in a bypass duct defined within a nacelle18, and also drives air along a core flow path C for compression andcommunication into the combustor section 26 then expansion through theturbine section 28. Although depicted as a two-spool turbofan gasturbine engine in the disclosed non-limiting embodiment, it should beunderstood that the concepts described herein are not limited to usewith two-spool turbofans as the teachings may be applied to other typesof turbine engines including three-spool architectures.

The exemplary engine 20 generally includes a low speed spool 30 and ahigh speed spool 32 mounted for rotation about an engine centrallongitudinal axis A relative to an engine static structure 36 viaseveral bearing systems 38. It should be understood that various bearingsystems 38 at various locations may alternatively or additionally beprovided, and the location of bearing systems 38 may be varied asappropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects, a first (or low) pressure compressor 44 and a first (orlow) pressure turbine 46. The inner shaft 40 is connected to a fansection 22 through a speed change mechanism, which in exemplary gasturbine engine 20 is illustrated as a geared architecture 48 to drivefan blades 42 at a lower speed than the low speed spool 30. The highspeed spool 32 includes an outer shaft 50 that interconnects a second(or high) pressure compressor 52 and a second (or high) pressure turbine54. A combustor 56 is arranged in exemplary gas turbine 20 between thehigh pressure compressor 52 and the high pressure turbine 54. Amid-turbine frame 58 of the engine static structure 36 may be arrangedgenerally between the high pressure turbine 54 and the low pressureturbine 46. The mid-turbine frame 58 further supports bearing systems 38in the turbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 58 includes airfoils 60 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and fan drive gear system 48 may be varied. For example,gear system 48 may be located aft of the low pressure compressor 44 andthe fan blades 42 may be positioned forward or aft of the location ofthe geared architecture 48 or even aft of turbine section 28.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than about ten (10), thegeared architecture 48 is an epicyclic gear train, such as a planetarygear system or other gear system, with a gear reduction ratio of greaterthan about 2.3 and the low pressure turbine 46 has a pressure ratio thatis greater than about five. In one disclosed embodiment, the engine 20bypass ratio is greater than about ten (10:1), the fan diameter issignificantly larger than that of the low pressure compressor 44, andthe low pressure turbine 46 has a pressure ratio that is greater thanabout five 5:1. Low pressure turbine 46 pressure ratio is pressuremeasured prior to inlet of low pressure turbine 46 as related to thepressure at the outlet of the low pressure turbine 46 prior to anexhaust nozzle. The geared architecture 48 may be an epicycle geartrain, such as a planetary gear system or other gear system, with a gearreduction ratio of greater than about 2.3:1 and less than about 5:1. Itshould be understood, however, that the above parameters are onlyexemplary of one embodiment of a geared architecture engine and that thepresent invention is applicable to other gas turbine engines includingdirect drive turbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and35,000 ft (10,668 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption(‘TSFC’)”—is the industry standard parameter of lbm of fuel being burneddivided by lbf of thrust the engine produces at that minimum point. “Lowfan pressure ratio” is the pressure ratio across the fan blade alone,without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressureratio as disclosed herein according to one non-limiting embodiment isless than about 1.45. “Low corrected fan tip speed” is the actual fantip speed in ft/sec divided by an industry standard temperaturecorrection of [(Tram ° R)/(518.7° R)]^(0.5). The “Low corrected fan tipspeed” as disclosed herein according to one non-limiting embodiment isless than about 1150 ft/second (350.5 meters/second).

The example gas turbine engine includes the fan section 22 thatcomprises in one non-limiting embodiment less than about 26 fan blades42. In another non-limiting embodiment, the fan section 22 includes lessthan about 20 fan blades 42. Moreover, in one disclosed embodiment thelow pressure turbine 46 includes no more than about 6 turbine rotorsschematically indicated at 34. In another non-limiting exampleembodiment, the low pressure turbine 46 includes about 3 turbine rotors.A ratio between the number of fan blades 42 and the number of lowpressure turbine rotors is between about 3.3 and about 8.6. The examplelow pressure turbine 46 provides the driving power to rotate the fansection 22 and, therefore, the relationship between the number ofturbine rotors 34 in the low pressure turbine 46 and the number ofblades 42 in the fan section 22 disclose an example gas turbine engine20 with increased power transfer efficiency.

The example engine 20 includes an accessory drive system 62 thatreceives power from both the high speed spool 32 and the low speed spool30. The accessory drive system 62 drives an accessory gearbox 68 thatincludes accessory component 72 and lubricant pump 74. The accessorycomponent 72 may include pumps, generators and other devices driven toenable operation of different engine and aircraft systems. The accessorygearbox 68 is also coupled to a starter 70. The starter 70 is capable ofdriving the accessory drive system 62 to start the engine 20. In thisexample, a tower shaft assembly 64 is coupled to both the low speedspool 30 and the high speed spool 32 to distribute power extractionbetween the two spools 30, 32.

Excessive power extraction from a single spool, such as the high speedspool 32, can limit operation and degrade overall performance and engineefficiency. Accordingly, the example accessory drive system 62 extractspower from both the low speed spool 30 and the high speed spool 32 tomeet the overall power demands of the engine 20 and the aircraftassociated with the engine.

Referring to FIG. 2, with continued reference to FIG. 1, the exampleaccessory drive system 62 includes a superposition gearbox 66 that iscoupled between accessory gearbox 68 and the tower shaft assembly 64.The superposition gearbox 66 is an epicyclic gearbox that includes a sungear 102 that rotates about an axis 118. A plurality of intermediategears 104 are engaged with the sun gear 102 and supported by a carrier106. A ring gear 108 circumscribes and is engaged with the plurality ofintermediate gears 104.

In the disclosed example, the tower shaft assembly 64 includes a firsttower shaft 76 that is driven by a gear 82 disposed on the low speedspool 30. A first gear 86 on the tower shaft 76 is coupled to the gear82. A second gear 88 is disposed on a second end of the tower shaft 76and engages a drive gear 90 disposed on a ring gear shaft 92.

A second tower shaft 78 is coupled to a drive gear 84 that is driven bythe high speed spool 32. The second tower shaft 78 includes a first gear94 driven by the gear 84 on the high speed spool 32. A second gear 96 ofthe second tower shaft 78 is engaged to drive gear 98 disposed on a sungear shaft 100. In this example, the first tower shaft 76 and the secondtower shaft 78 are disposed concentrically about a common axis 80.Moreover, the axis 80 is disposed at an angle relative to the enginelongitudinal axis A and an axis 112 of the superposition gearbox 66. Itshould be appreciated that although the specific disclosed embodimentincludes concentric tower shafts 76, 78, other configurations andorientations of the tower shafts are within the contemplation and scopeof this disclosure.

First tower shaft 76 is coupled to the ring gear shaft 92 that isselectively coupled to the ring gear 108. The second tower shaft 78 iscoupled to the sun gear shaft 100 that is coupled to drive the sun gear102. The sun gear shaft 100 is directly coupled to the sun gear 102 andextends past the sun gear 102 to the accessory gearbox 68.

The superposition gearbox 66, therefore, has a first input provided bythe first tower shaft 76 through the ring gear shaft 92 to drive thering gear 108 and a second input provided by the second tower shaft 78to drive the sun gear shaft 100 and, thereby, the sun gear 102. A firstoutput from the superposition gearbox 66 is provided by a carrier shaft110 that is coupled to the carrier 106. The carrier shaft 110 drivesportions of the accessory gearbox 68 in the disclosed exampleembodiment. The accessory gearbox 68 includes another gear system orplurality of gears as is required to the drive accessory componentsschematically illustrated at 72 and 74.

The sun gear shaft 100 provides a second output that extends through theaccessory gearbox 68 and is coupled to the starter 70. The starter 70provides a driving input to the sun gear 102 through the sun gear shaft100.

Referring to FIG. 3, with continued reference to FIG. 2, the examplesuperposition gearbox 66 includes a direct connection between thestarter 70 and the sun gear shaft 100 to provide for direct driving ofthe high speed spool 32. The sun gear shaft 100 is coupled to thestarter 70 through a starter clutch 112. The starter clutch 112 in thisexample is a mechanical one-way clutch that enables direct driving ofthe high speed spool 32 during starting operations. Once the high speedspool 32 is operating, the starter clutch 112 prevents back driving orover driving of the starter 70. The sun gear shaft 100 is directlyconnected to the starter rather than being driven through a gear system.The direct drive of the high speed spool 32 through the directconnection simplifies operation and the mechanical connections.

Moreover, the example superposition gearbox 66 provides the first outputthrough the carrier shaft 110 that drives gear systems 122, 124 that inturn drive the accessory components 72, 74. Once the engine is started,the first output through the carrier shaft 110 provides the drivinginput required to power the accessory components 72, 74.

The superposition gearbox 66 includes a first clutch 114 that couplesthe ring gear shaft 92 to the ring gear 108. A second clutch 116 couplesthe ring gear 108 to a static engine structure 36. In this example, boththe first clutch 114 and 116 are mechanical one-way clutches. Moreover,in this example, the first and second mechanical one-way clutches 114,116 are sprag clutches. It should be appreciated that although spragclutches are disclosed by way of example, other mechanical clutchsystems could be utilized and are within the contemplation of thisdisclosure.

The second clutch 116 couples the ring gear 108 to the engine staticstructure 36 during a starting operation to prevent reverse rotation ofthe ring gear 108 and thereby the first tower shaft 76 and the low speedspool 30. When the ring gear 108 is fixed, the starter 70 will drive thesun gear shaft 100 such that it will be the only driving output back tothe high speed spool 32.

During a starting operation, the starter 70 will drive the sun gearshaft in a first direction as is indicated by arrows 126. The starterclutch 112 will engage and enable driving of the sun gear shaft 100 bythe starter 70. The same rotation provided by the starter 70 will engagethe ring gear 108 such that the second clutch 116 will lock the ringgear 108 to the static structure to prevent rotation of the ring gear108. The first clutch 114 is not locked in this direction, but does notreceive a driving input and therefore does not rotate the correspondingfirst tower shaft 76. The starter 70 thereby directly drives the highspeed spool 32 to start the engine.

Once the engine has started and the high speed spool 32 is rotating atspeed, the starter 70 will be stopped. The high speed spool 32 willbegin driving the second tower shaft 78 and thereby the sun gear shaft100. The higher speed provided by the driving of the high speed spool 32will disengage the starter 70. Additionally, once the low speed spool 30begins operation, the first tower shaft 76 will being rotating. Thefirst clutch 114 will then engage to couple the ring gear shaft 92 tothe ring gear 108. In this operating condition, both the low speed spool30 and the high speed spool 32 will drive portions of the superpositiongearbox 66. The first clutch 114 will couple power from the first towershaft 76 to the ring gear 108. The second clutch 116 will free wheel andallow rotation of the ring gear 108. The second tower shaft 78 willdrive the sun gear 102 and thereby the intermediate gears 104 and thecarrier 106. The carrier 106 will in turn drive carrier shaft 110 todrive the gear systems 122, 124 within the accessory gearbox 68. Powerfrom each of the high spool 32 and the low spool 30 will be split todrive the carrier shat 110 and power the accessory components 72 74throughout engine operation.

The example accessory drive system 62 includes a superposition gearbox66 that automatically distributes input driving torque between the lowspeed spool 30, the high speed spool 32 and the accessory gearbox 68 asrequired during engine operation. The selective operation of thesuperposition gearbox 66 is enabled by first and second one-way clutchesthat provide different combinations of inputs and outputs thatautomatically couple based engine operating conditions. Moreover, thedirect starting input from the starter is enabled by the coupling of thestarter directly to the sun gear shaft along with fixing of the ringgear by a one-way mechanical clutch.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that this disclosure is not just amaterial specification and that certain modifications would come withinthe scope of this disclosure. For that reason, the following claimsshould be studied to determine the scope and content of this disclosure.

What is claimed is:
 1. A turbofan engine comprising: a first spoolincluding a first turbine; a second spool including a second turbinedisposed axially forward of the first turbine; a first tower shaftengaged to the first spool; a second tower shaft engaged to the secondspool; a superposition gearbox including a sun gear, a plurality ofintermediate gears engaged to the sun gear and supported in a carrierand a ring gear circumscribing the intermediate gears, wherein thesecond tower shaft is engaged to drive the sun gear; a starterselectively coupled to the sun gear through a starter clutch; a firstclutch for selectively coupling the first tower shaft to the ring gear;a second clutch for selectively coupling the ring gear to a staticstructure of the engine; and an accessory gearbox driven by an output ofthe superposition gearbox.
 2. The turbofan engine as recited in claim 1,wherein the output of the superposition gearbox comprises a carriershaft attached to the carrier.
 3. The turbofan engine as recited inclaim 2, wherein the superposition gearbox includes the carrier shaftattached to the carrier to drive gear systems within the accessorygearbox and a sun gear shaft that is coupled to the sun gear, the secondtower shaft and the starter through the starter clutch.
 4. The turbofanengine as recited in claim 1, wherein the first turbine comprises a lowpressure turbine and the second turbine comprises a high pressureturbine.
 5. The turbofan engine as recited in claim 1, wherein the firsttower shaft and the second tower shaft are concentric about a commonaxis.
 6. The turbofan engine as recited in claim 1, wherein the firsttower shaft and the second tower shaft are disposed about differentaxes.
 7. The turbofan engine as recited in claim 1, wherein the starterclutch, first clutch and the second clutch comprise one-way mechanicalclutches.
 8. The turbofan engine as recited in claim 1, wherein thefirst clutch couples the first tower shaft to the ring gear and thesecond tower shaft drives the sun gear in a first operating conditionsuch that both the first tower shaft and the second tower shaft combineto drive the output.
 9. The turbofan engine as recited in claim 8,wherein the second clutch couples the ring gear to the static enginestructure and the starter clutch couples the starter to the sun gear andthe first clutch couples the first tower shaft to the ring gear suchthat starter drives the sun gear and thereby the second tower shaft andthe second spool in a starting operating condition.
 10. The turbofanengine as recited in claim 9, wherein the second clutch decouples thering gear from the static structure and couples the first clutch inresponse to rotation of the first tower shaft driven by the first spool.11. An auxiliary gearbox drive system for a turbofan engine, theauxiliary gearbox drive system comprising: a sun gear, wherein the sungear is coupled to a second tower shaft of the turbofan engine; aplurality of intermediate gears engaged to the sun gear and supported ina carrier; a ring gear circumscribing the intermediate gears; a starterselectively coupled to the sun gear through a starter clutch; a firstmeans for selectively coupling the ring gear to a first tower shaft ofthe turbofan engine; a second means for selectively coupling the ringgear to a fixed structure; and an output to an auxiliary gearbox. 12.The auxiliary gearbox as recited in claim 11, wherein the output to theauxiliary gearbox comprises a carrier shaft attached to the carrier. 13.The auxiliary gearbox as recited in claim 12, wherein each of the firstmeans for selectively coupling the ring gear to the first tower shaftand the second means for selectively coupling the ring gear to the fixedengine structure are one-way mechanical clutches.
 14. The auxiliarygearbox as recited in claim 12, wherein the first means couples thefirst tower shaft to the ring gear and the second tower shaft drives thesun gear in a first operating condition such that both the first towershaft and the second tower shaft combine to drive the carrier shaft. 15.The auxiliary gearbox as recited in claim 14, wherein the second meanscouples the ring gear to the fixed structure when the starter is drivingthe sun gear to rotate an engine spool through the second tower shaft ina starting operating condition.
 16. A method of operating an auxiliarygearbox for a turbofan engine comprising: coupling a first tower shaftto engage a first spool; coupling a second tower shaft to engage asecond spool; coupling a sun gear of a superposition gearbox to thesecond tower shaft, wherein the superposition gearbox includes the sungear, a plurality of intermediate gears engaged to the sun gear andsupported in a carrier and a ring gear circumscribing the intermediategears; selectively coupling a starter to the sun gear; selectivelycoupling the ring gear to an engine static structure; driving the secondspool with the starter through the sun gear to rotate the second spooland start the turbofan engine; and coupling the first tower shaft to thering gear such that the first tower shaft drives the ring gear and thesecond tower shaft drives the sun gear once both the first spool and thesecond spool are rotating independent of rotation of the starter suchthat both the first tower shaft and the second tower shaft combine todrive an output shaft from the superposition gearbox.
 17. The method asrecited in claim 16, wherein the output shaft is attached to the carrierand drives gear systems disposed within an accessory gearbox.